The present invention relates to a method for determining phase offsets in a complex-valued image acquired in Magnetic Resonance Imaging (MRI).
MRI is used in radiology to visualize details of structures in a patient's body. To align the magnetic spin of the nuclei, mostly of protons in water molecules in the body tissue, the patient is placed inside a powerful static magnetic field. Excited by an electro-magnetic radio-frequency pulse from a transmitter coil, the nuclei resonating at this frequency deflect and then gradually relax towards the static field while emitting detectable electro-magnetic radiation, which can be captured as an “echo” at a certain time after excitation (the “echo time”) by a receiver coil. Relaxation times and the resonance frequency of the nuclei depend both on local properties of the tissue material, which represents the underlying principle allowing visualization of these properties. The characteristics of the image are also influenced by proton density and magnetic field strength. By superposing linear magnetic field gradients in three orthogonal directions and adjusting the excitation frequency, certain volumetric regions (“volume elements”) can be measured by both selective excitation and frequency analysis of the captured echo signals. Fourier-transforming the latter generates complex representations of the captured values in image space and their phases, which can be made readable.
When the excitation is performed on 2-dimensional slices through the object, they are selected by adjusting the field gradients, thereafter resonant frequency values are captured and processed, forming 2-dimensional images, a number of which can be acquired to form a 3-dimensional representation of the object. Alternatively, in 3-dimensional imaging, a large volume of tissue is excited and spatially encoded using frequency encoding in one direction and phase encoding in both of the remaining two orthogonal directions.
In the past, magnitude values have been used primarily from the complex representations. Nevertheless, phase values allow for the extraction of additional information about local properties of the tissue, where they specifically benefit from strong susceptibility effects at high magnetic field strengths. For example, phase information is used in neuro-imaging in phase-contrast angiography, Susceptibility-Weighted Imaging (SWI), susceptibility mapping—also known as Quantitative Susceptibility Mapping (QSM), Susceptibility Tensor Imaging, to depict iron accumulation in neurodegenerative disorders and to map in vivo conductivity. It can also be used to monitor temperature and encode flow velocity.
However, each phase value acquired by a receiver coil of the MRI machine is subject to a time-independent offset, often referred to as “phase offset”. The phase offset comprises spatially constant components, e.g. due to the cable length from a receiver coil to a receiver, as well as spatially variable components, e.g. due to the path lengths of the excitation and echo signals from particular locations in the object to the receiver coil in question.
It has been an aim of research to determine phase offsets and subsequently eliminate their effects on Magnetic Resonance Imaging, e.g., in order to facilitate combining multiple phase images acquired with a plurality of receiver coils arranged in an array around the object and thereby increase the quality of an acquired image in terms of its signal-to-noise ratio (SNR). Several approaches have been presented in the past to determine—or at least roughly estimate—and eliminate phase offsets, whereupon a combined phase image can be generated:
One of these approaches, proposed by Roemer, P. B. et al., “The NMR phased array”, Magn Reson Med 1990, 16; pp. 192-225, uses an additional body coil or other homogeneous volume reference coil, i.e., a coil which is separate from said receiver coils and has to be sensitive over (at least) all the tissue over which the receiver coils, arranged in an array around the object, are sensitive, for referencing and using a phase offset measured separately by means of the body coil for each receiver coil; however, such an additional reference coil is not commonly available in ultra-high field scanners and requires extra space and control. Moreover, inhomogeneities of the reference coil, inevitable in ultra-high field scanners, are introduced into the phase which consequently suffers both from field inhomogeneities and from the offset from the reference coil.
A different approach, presented by Hammond, K. E. et al., “Development of a robust method for generating 7.0 T multichannel phase images of the brain with application to normal volunteers and patients with neurological diseases”, NeuroImage 2008, 39; pp. 1682-1692, suggests to estimate a spatially constant phase offset by setting the phase values to zero in all coils at the centre of an image. This method, while being easy to apply, results in areas of poor phase matching.
An alternative solution is to refer the phase values of each receiver coil to a “virtual reference coil” which is the result of a two-step procedure. In the first step, a combined image (the Virtual Reference Coil, or VRC, image) is generated using an image-based constant (as in the method of Hammond et al.). In the second step, the phase image from each coil is referenced to the VRC image. While the matching of phase values of different receiver coils is very good in this case, the method cannot separate the phase offset and magnetic inhomogeneity-related contributions to the total phase.
According to yet another solution Robinson, S. et al., “Combining phase images from multi-channel RF coils using 3D phase offset maps derived from a dual-echo scan”, Magn Reson Med 2011, 65; pp. 1638-1648, propose to unwrap the first and second phase images, acquired at the first and the second echo time, respectively, and calculate a phase difference image therefrom. The phase difference image is then unwrapped and added to the unwrapped first phase image to yield an estimate of the phase at the second echo time. By the differences between said estimate and the acquired second phase image, further wraps are identified and the first and the second phase images are further unwrapped to calculate the phase offsets for each coil therefrom. While this method achieves very high SNR and contrast, computing time and storage requirements are also high due to (repeated) unwrapping.